How Downstream Plasma Cleaning Works

Problem: Users in EM facilities with several current generation tools including FE SEM and dual beam FIB/SEM will, despite taking precautions to insert only clean specimens, sometimes get contamination introduced into their microscopes. This manifests itself as a dark rectangle on areas which have been exposed to the incident beam. How can this be prevented? Can it be removed from the previously exposed samples?

Solution: Even minute amounts of hydrocarbon contamination can be precipitated onto the sample surface when exposed to the energetic electron or ion beam. Routine sample cleaning before insertion helps, but inevitably the interior of the instrument gets contaminated and requires cleaning. Downstream plasma cleaning has been shown to be very effective at removing these contaminants.

An RF plasma product that used the technique of secondary or downstream plasma cleaning to address the problem of cleaning internal surfaces of the vacuum chambers in electron microscopes was patented and introduced in 1999 (XEI Scientific). The product is called the Evactron® De- Contaminator, and a schematic of its downstream plasma process is shown here.

This type of system produces the active plasma in a remote chamber (called a Plasma Radical Source or PRS) and transfers the active species to the cleaning chamber via gas flow relying primarily on the chemical activity of the reactive radicals produced by the plasma. Experiments with different gases to create the plasma have shown room air to be an excellent source of oxygen to create reactive radicals and efficiently crack hydrocarbon molecules. It has the benefits of being available, free, and safe. Also, via the choice of other noncorrosive gases for producing radicals, different chemical etch processes may be selected and benign regimes for sensitive components may be obtained as well as optimized chemistries for the fast removal of unwanted contaminants.

While the energetic ions are contained in the external Plasma Radical Source, reactive gas radicals are allowed to drift through the vacuum chamber and come into contact with the sample and internal surfaces. Photons in the plasma are in the Vacuum UV (VUV) wavelengths, and VUV energy is very effective in breaking most organic bonds, i.e., C-H, C-C, C=C, C-O, and C-N. Thus, high molecular weight contaminants are broken into smaller components. A second cleaning action is carried out by the various oxygen species created in the plasma (O2 +, O2 -, O3, O, O+, O-, ionized ozone, meta-stably-excited oxygen, and free electrons) which combine with organic contaminants to form H2O, CO, CO2, and low molecular weight hydrocarbons. Exhibiting relatively high vapor pressure, these compounds are easily pumped out of the microscope by the vacuum system. The downstream plasma technique has proven extremely useful and is now well accepted. There are over 1,400 installations of the XEI tool on nearly all makes and models of SEM and Dual Beam FEB/SEMs. And with this system being portable and easily moved among several columns, a modest investment can provide the solution to the entire lab’s contamination problem.